Nitto 55G2's are a nice economical tire that perform slightly better than the Goodyear Eagle F1's. If you want a better performing, more expensive tire, Toyota Proxes or Nitto NT05's.
I was going to say the driveshaft, wheels or tires.
If you're 100% sure it's not the driveshaft, then go to the wheels. Are you sure those American Muscle wheels are 100% round? Not sure about the quality of those wheels, but I may be wrong.
Good luck man and keep us posted to what you find out.
I wouldn’t take what Costco says as the gospel. Tires are much cheaper to replace than rims.
Make sure you take pictures of your wheels before you get new tires.
Also make sure you walk with the service adviser around the car to look at the wheels.
That way when they damage them you'll at least have proof... ask me how I know.